This application concerns presses such as trim presses used in thermoforming products to cut the individual molded parts free from the plastic sheeting in which the parts have been thermoformed.
Such presses are designed to be used to trim different parts by changing tools mounted in the press.
A movable platen has a tool mounted thereto, and is typically reciprocated by a rotating pair of flywheels, the flywheels driven by a motor with interposed gearing or belt drives. The flywheels drive the movable platen by means of a pair of connecting arms, each driven by a respective flywheel. Each connecting arm has a crank pin housing at one end having a bore receiving a crank pin secured to the flywheel at an off center location. An opposite end of each connecting arm is pivotally connected to one side of the movable platen. Rotation of the flywheels causes stroking of the movable platen by the connecting arms, which causes the tool on the platen and a mating stationary tool to cut thermoformed parts free from the sheet.
The sheet with the parts formed thereon is fed into the press step by step, advanced after each cycle of the press to bring the next set of parts to be trimmed from the sheet into alignment with the tool.
In order to trim a different part the tool for a given part must be removed from the press and a different tool to be used with each different part installed in its place.
The tools are typically complex and bulky, having ejectors, take off tubes, etc., installed thereon which make it difficult to change tools.
The tool often must be partially disassembled while still in the press in order to be removed, since the movable platen cannot be retracted sufficiently to create a enough clearance to remove the tool with the attachments on the tool still installed.
This process is very time consuming and results in an extended shutdown of the trim press, as much labor is required to accomplish a tool changeover if the tool must be at least partially disassembled.
As noted above, it is also sometimes necessary to perform maintenance on the tool such as sharpening the cutting edges, and the small clearances available when the platen is fully retracted often makes maintenance on the tool while in the press difficult or impracticable.
Another difficulty is encountered in the changing of the stroke of the press, which often must be done when a different part is to be trimmed. This requires a radial relocation of the connection of each crank pin assembly on an associated flywheel either closer to its center or further away. Since two crank pins are present, each on a respective flywheel, the crank pins must be completely removed from the associated connecting arm. This is because the movable platen is mounted on linear bearings and shifting a crank pin to adjust the stroke would tend to cause movement of the movable platen by the connecting arm since it is connected to the crank pin. If only one crank pin is adjusted at a time, binding of the platen on the linear bearings would result. Thus, the crank pins must usually be completely detached and then installed at the new location so as to avoid binding of the platen.
A time consuming complete detachment of each of the crank pins from the associated connecting arm is required to change the press stroke by the conventional practice.
Such presses also often have counterweights which are driven by the press in the opposite direction from the movable platen to balance out the momentum of the platen and thereby allow higher speeds of operation by minimizing movement of the press caused by reaction forces generated by reversing movement of the movable platen and tool.
However, changing the stroke of the movable platen can result in causing the platen momentum to be unbalanced by the counterweight motion, limiting operational speeds and causing rapid wear of some of the mechanical components.
Accordingly, it is an object of the present invention to provide a press which quickly and easily allows a temporary greater extent of retraction movement of the movable platen so as to be sufficient to create a clearance adequate to accommodate the removal of a complete tool with accessories attached; and, to also allow maintenance to be carried out on the tool while it is still in place in the press.
It is a further object of the present invention to provide an arrangement for adjusting the stroke of a press in which the crank pins do not have to be detached from the connecting arms in order to shift the location of their attachment to the flywheel to change the press stroke.
It is another object of the invention is to provide a counterweight system which has a counter movement which is changed correspondingly when a change in the stroke of the press is carried out.